One of the most fundamental observables in testing our understanding of gravity is the speed with which gravity propagates.
In a Lorentz-invariant solution gravitational waves will propagate at the speed of light, but this need not be the case if Lorentz invariance is spontaneously broken, as it is in our Universe.
Here, just as for any other waves, the speed of gravitational waves \(c_{\rm GW}\)  depends on the medium they are propagating through. Measuring \(c_{\rm GW}\)  therefore lets us probe the nature of this medium and hence the particle content of the Universe.
With dark energy accounting for close to 70% of the energy budget of our current Universe [1]}, constraining \(c_{\rm GW}\)  provides an especially powerful tool in hunting for new particles potentially associated with dark energy. These new particles are a generic consequence of departures from modelling dark energy as a cosmological constant \(\Lambda {}\) . Tight constraints found on some such theories [2]}, [3]}, [4]}, [5]} – also see earlier related work [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]} – following the joint observations of GW170817 and GRB 170817A [17]}, [18]}, [19]}, [20]}, [21]} neatly illustrate this point.
